A communications system from which at least one way, and usually two way, communications between a transmitter that may be part of a first transceiver and a receiver that may be part of a second transceiver. In many communications systems, a transconductance amplifier as part of a modulator is used to translate an input voltage to an output current. The voltage-to-current transformation should be linear over the frequency range of operation of the transconductance amplifier. The design of voltage-to-current transconductance amplifiers that maintain linearity over a wide operating frequency range has been very difficult.
Wideband noise generated by the transconductance amplifier, and other circuits in a communication system, may be upconverted down-stream of the transconductance amplifier and can cause interference with a receiver. To prevent interference with a receiver, the modulator output signal is often filtered to reduce noise generated by the modulator in the receive band. Once upconverted, the noise that must be filtered is separated from the desired transmit signal by a relatively small spatial difference in frequency, necessitating a high quality factor filter to achieve the filtering. High frequency, high quality factor filters are not readily implemented on an integrated circuit due to the lack of high quality factor inductors. Instead, a filter external to the integrated circuit is typically employed to reduce the wideband noise in the receive band to an acceptably low level. Such a filter can be placed either prior to the signal being provided to a power amplifier, or subsequent to the signal being amplified by the power amplifier. The upconverted signal is passed through a power amplifier to drive the antenna. Filtering after amplifying has the drawback that in addition to filtering the noise, the desired signal will be attenuated slightly due to the insertion loss of the filter, and the power amplifier must compensate for this loss by providing additional power. This is especially undesirable for battery powered devices.
What is needed is a highly linear transconductance amplifier with improved wideband noise filtering characteristics that can be readily implemented on an integrated circuit. Such a transconductance amplifier would either eliminate the need for a separate high Q filter downstream of the transconductance amplifier, or it would reduce the filtering requirements on the downstream filter resulting in a lower cost filter.